Looms usually possess devices for conveying the weft yarn or yarns and devices for handling the yarns forming the warp, known as dobbies, which move in synchronism with the former to form the desired woven design.
During weaving, it is possible for breakages of the weft yarn to occur and in such cases, for the purpose of preventing a defect from occurring in the finished piece, it is necessary to interrupt weaving and, while holding the devices for conveying the weft yarn stationary, to actuate the dobby in reverse thus unpicking the woven fabric formed from the moment of rupture to the moment where the loom was stopped, thereby enabling continuity of the interrupted weft yarn to be restored.
For this purpose it is then necessary to separate the dobby from the loom and to effect its reverse movement with an auxiliary motor device, operating at low speed, as far as the desired position.
In order to start weaving again, it is therefore necessary to restore the coupling between loom and dobby, by bringing them into the relative angular position corresponding to the instant at which they were separated, thereby ensuring synchronism between the relative movements in the manner in which it existed before the interruption.
For this purpose, couplings having two clutches are in use, such devices permit the loom to be coupled to the dobby by the first clutch and enable them to be separated in order to connect the dobby, by means of the second clutch, to an auxiliary drive device, by means of which the independent motion of the dobby itself can be obtained.
The coupling between loom and dobby in the correct angular position for starting weaving again is assured by the presence of asymmetrical members in the respective clutch, which permit the closure of the clutch itself only in the desired position.
Couplings of this class are known, for example, from German Pat. No. 2,509,665, German Pat. No. 2,222,151, French Pat. No. 2,520,762, European Pat. No. 0,086,999, British Pat. No. 649,032, and U.S. Pat. Nos. 3,175,665, 3,390,749, 4,428,404 and 4,592,392.
Among the foregoing, the German Pat. No. 2,509,665 (LENTZ) discloses a search mechanism for the weft for looms, which comprises two clutches, of which one is interposed between a driving component and a driven component of the principal shaft of the machine, while the second clutch is interposed between the driven component and an auxiliary motor, for actuating the driven component in reverse motion German Pat. No. 2,222,151 (NUOVO PIGNONE), discloses a search mechanism for the weft for looms, in which there is an electromagnetic clutch and an electromagnetic brake U.S. Pat. No. 4,428,404 (Kleiner, assigned to STAUBLI Ltd.) discloses a low-velocity search mechanism for the weft for looms, which is composed of two mechanically actuated clutches for coupling a driven shaft to a motor shaft or to an auxiliary motor, these being arranged coaxially and capable of being engaged independently with separate controls U.S. Pat. No. 3,390,749 (Pospisil, Boucek, Elias, assigned to VYZKUMNY USTAV BAVLNARSKY) has a double clutch, with independent electromagnetic actuation of each clutch.
Furthermore, U.S. Pat. No. 4,592,392 (Wandeweghe-Derde, assigned to N. V. WEEFAUTOMATEN PICANOL) has a search mechanism for the weft for looms, in which, as for the preceding mechanisms, the drive shaft and the driven shaft are connected by a first clutch, having springs with electromagnetic disengagement, while a second clutch, electromagnetically actuated, connects the driven shaft to the auxiliary motor device this coupling differs substantially from these described earlier by its mechanical arrangement, which provides the presence of an axially fixed, central element, linked to the end of the driven shaft, which carries at opposite sides fixed clutch elements for the controlled rotational coupling with it of corresponding, axially slidable clutch elements, rotatably connected respectively to the driving shaft and to transmission elements coupled with the auxiliary motor device.
Such a mechanism possesses, in its construction, certain limitations and disadvantages, resulting from the fact that one of the clutches, intended for coupling the drive shaft with the driven shaft, has the rotor only partly surrounding the pole piece of the electromagnet, whereas the armature of the clutch extends externally of the electromagnet to complete the magnetic circuit.
The other of the clutches, intended for coupling the driven shaft with the auxiliary motor device, has its rotor, surrounding the pole piece of the associated electromagnet, intended for axial translatory movement for passing from the closed clutch position to the open clutch position. In both cases, therefore, the members intended for forming the magnetic circuits of the relevant electromagnets are also required to carry out axial movements. This means that these components must be constructed with sufficient play to permit the axial sliding intended for engagement and disengagement, so that it is necessary to prevent such movements from being obstructed by friction and the like. However, the clearances required for this purpose give rise to a large air gap, in the radial direction, between the elements of the magnetic circuit and to irregular variations in this air gap, producing losses in the value of the attraction force of the electromagnets and consequent irregularities in operation.
The rotor of the clutch for connecting the driven shaft with the auxiliary motor is also subjected to axial sliding on the surface of the respective support bearing. On the one hand, this necessitates radial clearances in the coupling of the bearing, with the aforementioned disadvantages associated with the increase in the magnetic air gap, and on the other hand such sliding movement can lead to friction and crawling, or to contact between rotor and electromagnet, with dispersion in the magnetic field generated.
Furthermore, said rotor, being subjected to movements in the axial direction between the engagement and disengagement positions, can be rotatably coupled to the auxiliary motor device only by means of a transmission which permits the axial displacement, such as a chain, belt or straight-toothed cylindrical gear, so that transmissions which require constancy in the positions of the coupled devices are precluded. These include transmission having gears with converging or skew axes, including the type with worm and helical gear wheel, which would offer in particular the advantage of directly achieving a high reduction ratio, without the adoption of further reduction units being necessary.
There is also known, from German Pat. No. 32 29 192, in the name of ZAHNRADFABRIK FRIEDRICHSHAFEN AG, a toothed clutch which possesses a rotor rotatable inside a fixed electromagnet, with an axially movable armature extending laterally outside the electromagnet, closing the magnetic circuit onto the electromagnet. The armature carries a set of teeth on its face capable of engaging with the corresponding set of teeth of a driven ring and it is pressed onto the ring by springs placed between the rotor and the armature.
This construction is in every way similar to that of the first of the of the N. V. WEEFAUTOMATEN PICANOL patent clutches already cited, and is intended for coupling the driving shaft to the driven shaft, which possesses, as stated, the rotor only partly surrounding the pole piece of the electromagnet, whereas the armature of the clutch extends externally of the electromagnet to complete the magnetic circuit. The clutch thus possesses the disadvantages already cited, deriving from the need to use a large air gap between the fixed electromagnet and the armature, in order to ensure the axial sliding of the latter without crawling.